An organic electroluminescence (EL) device is a self-luminous device utilizing such a principle that a fluorescent substance emits light with recombination energy of holes injected from an anode and electrons injected from a cathode on application of an electric field. An organic EL device containing an organic material as a constitutional material has been actively investigated since C. W. Tang, et al., Eastman Kodak Corporation, reported a low voltage driving organic EL device with a stacked device (C. W. Tang and S. A. Vanslyke, Applied Physics Letters, vol. 51, p. 913 (1987), etc.).
For example, Patent Documents 1 to 4 disclose a diamine compound having a fluorene skeleton between two nitrogen atoms, and disclose the use of the diamine compound as a material of a hole transporting layer that is adjacent to a light emitting layer, by which crystallization of the hole transporting material due to heat generation or the like on light emission in the light emitting layer is suppressed, and thus an organic EL device is provided that is improved in stability and durability as compared to a diamine compound having a biphenylene group between two nitrogen atoms or a monoamine compound having a fluorene skeleton.
Patent Document 5 discloses the use of a diamine compound having two nitrogen atoms bonded through a biphenylene group as a material of a first hole transporting layer, and the use of an aromatic amine derivative having a dibenzofuran structure and a carbazole structure as a material of a second hole transporting layer that is adjacent to a light emitting layer, and thus an organic EL device is provided that has a low driving voltage and a long service life. Patent Document 6 discloses the use of a diamine compound having two nitrogen atoms bonded through a biphenylene group in a first hole transporting layer, and the use of an amine compound having a particular hetero aryl structure in a second hole transporting layer, in a phosphorescent organic EL device, by which the second hole transporting layer has electron blocking property, electroresistance, and hole injecting and transporting property, and thus an organic EL device is provided that has a high efficiency and a long service life. Patent Document 7 discloses the use of a compound having a carbazole ring structure in a hole transporting layer that is adjacent to a light emitting layer, and thus an organic EL device is provided that has a high light emitting efficiency and a low driving voltage.
In summary, an organic EL device, particularly a phosphorescent device, has been enhanced in the capability of the device in such a manner that the hole transporting layer has a two-layer structure including the first hole transporting layer and the second hole transporting layer, and a highly functional material is used in the second hole transporting layer that is adjacent to the light emitting layer.
The capabilities that are required for the second hole transporting layer are as follows: (i) the second hole transporting layer has higher triplet energy (preferably 2.6 eV or more) for preventing the excitation energy of the phosphorescent light emitting layer from being diffused; (ii) the second hole transporting layer has electroresistance since the layer is adjacent to the light emitting layer; (iii) the second hole transporting layer is an organic layer that has a small affinity (preferably 2.4 eV or less) for preventing electrons from leaking from the light emitting layer; and (iv) the second hole transporting layer is an organic layer that has a large ionization potential (preferably 5.5 eV or more) for accelerating injection of holes to the light emitting layer. As a material satisfying these requirements, a molecular skeleton with high electroresistance having a triphenylamine skeleton and bonded thereto a hetero aryl ring, such as carbazole and dibenzofuran has been preferably used.
The first hole transporting layer is generally demanded to be excellent in the hole injection property to the second hole transporting layer.
It has been considered that a compound having a nature of a p type semiconductor (which may also be referred to herein as an acceptor material) is contained in the hole injecting layer for enhancing the hole injection property (see Patent Documents 8 and 9).
In view of the progress of research and development of the organic EL devices mentioned above, it is necessary in a commercial device that light that is internally emitted is taken out to the outside of the device with high efficiency for each of the light emission colors of the organic EL device. Accordingly, it is necessary to control the optical path length of the total device by controlling the thickness of the hole transporting layer having a higher carrier transporting property than the other organic layers. Under the present situation, therefore, a hole transporting material is demanded that has a high mobility to such an extent that the driving voltage may not be increased even when the thickness of the hole transporting layer is increased, and it is also demanded that a hole transporting material having a large carrier formation amount through interaction with an acceptor material, and the hole transporting material is used in the first hole transporting layer.